Phase 10: Per-User Rate Limits — messages + tokens, quota UI, admin usage

Backend:
- max_ai_messages_per_day + max_ai_tokens_per_day on User model (nullable, override)
- Migration 008: add columns + seed default settings (100 msgs, 500K tokens)
- usage_service: count today's messages + tokens, check quota, get limits
- GET /chats/quota returns usage vs limits + reset time
- POST /chats/{id}/messages checks quota before streaming (429 if exceeded)
- Admin user schemas expose both limit fields
- GET /admin/usage returns per-user daily message + token counts
- admin_user_service allows updating both limit fields

Frontend:
- Chat header shows "X/Y messages · XK/YK tokens" with red highlight at limit
- Quota refreshes every 30s via TanStack Query
- Admin usage page with table: user, messages today, tokens today
- Route + sidebar entry for admin usage
- English + Russian translations

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/schemas/admin.py

@@ -14,6 +14,8 @@ class AdminUserResponse(BaseModel):
     role: str
     is_active: bool
     max_chats: int
+    max_ai_messages_per_day: int | None
+    max_ai_tokens_per_day: int | None
     created_at: datetime
 
     model_config = {"from_attributes": True}
@@ -26,12 +28,16 @@ class AdminUserCreateRequest(BaseModel):
     full_name: str | None = None
     role: Literal["user", "admin"] = "user"
     max_chats: int = 10
+    max_ai_messages_per_day: int | None = None
+    max_ai_tokens_per_day: int | None = None
 
 
 class AdminUserUpdateRequest(BaseModel):
     role: Literal["user", "admin"] | None = None
     is_active: bool | None = None
     max_chats: int | None = None
+    max_ai_messages_per_day: int | None = None
+    max_ai_tokens_per_day: int | None = None
     full_name: str | None = None